28 August 2020

CIS Navy VLF 50Bd/75 FSK (T-600 75Hz)

50Bd/75 FSK is the T600 waveform variant utilized by Russian Navy on 18.1 kHz, commonly for submarine communications.

Fig. 1 - 50Bd/75 waveform
As seen in CIS 36-50, frames are constructed from data blocks consisting of 7-bit words with a fixed ratio of '1's vs. '0's of 4 to 3 or vice versa, depending on polarity of reception (Figure 2). 

Fig. 2 - 4/3 ratio test on a five-message transmission (s1 and s2 data blocks are the same)
Multiple Russian military naval communication stations share this frequency (18.10 KHz) and the call sign "RDL".  The 24h transmission schedule has frequent flash-override messages in A1A Morse, FSK-Morse and T600 75 Hz, as shown in the lower image of Figure 1.

https://yadi.sk/d/0M7f_H_WOD9LTA (T600-75 bitstream)

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